In recent years, contents delivery services have been widely used and such services are mostly realized by a mechanism in which a server that receives a request from a client delivers contents to the client. In this mode, there is a problem that the server load increases with an increasing number of clients (users). Such a problem may effectively be solved by using a P2P-type delivery service that exchanges data between user nodes. A node in an application concerned is a terminal such a personal computer and communication instrument connected to a network and a link in the network connects nodes. For example, ‘BitTorrent’ is an example of the P2P-type delivery service. FIG. 32 depicts an example of the configuration of a P2P-type delivery system using conventional ‘BitTorrent.’
As depicted in FIG. 32, for example, a node in a network is assigned a role of managing nodes in ‘BitTorrent.’ The node that manages nodes will be called a management node. A node in the network acquires contents according to procedures (1) to (3) depicted in FIG. 32.
That is, procedure (1): Node E accesses the management node of contents A to receive information about a node having the whole or a portion of the contents from the management node. Procedure (2): the management node adds, to node information, information about the node (Node E) that makes a request. Procedures (3): Node E accesses another node such as Node A or Node D (having the contents) based on the received information to receive the contents.
Here, a provider of contents may preferably delete or update contents once delivered. In such a case, in a system in which a server delivers contents collectively, contents are deleted or updated by the server so that deleted contents or contents of an older version are not delivered thereafter. In a P2P mode system, however, contents are transmitted and received between nodes and therefore, even if an update or the like is performed by the server, there is a possibility that deleted contents or contents of an older version are delivered thereafter.
In addition, contents held by a node that receives contents may be preferably deleted or updated so that the original contents are not used.
For example, according to Japanese Patent Application Laid-Open No. Hei05-274210 as a publicly known technology concerning file sharing, a technology that may reduce the number of times of access to a network and the amount of transfer data in the network in which a file of a file server is shared by issuing an access request to the file server when a node does not have the file cached in a local machine is discussed.